Femtosecond Diffraction Studies of Shock and Ramp-Compressed Matter

Lead Research Organisation: University of Oxford
Department Name: Oxford Physics

Abstract

The project aims to use the femtosecond pulses from a hard x-ray free electron laser (the Linac Coherent Light Source - LCLS at SLAC) to measure deformation and structure of matter shock or ramp compressed to multi-megabar pressures, accessing regions of phase space inaccessible to study with diamond anvil cells. The experiments will be augmented with multi-million atom molecular dynamics simulations coupled with diffraction simulations. The research has both applied and fundamental aspects. From the fundamental point of view a significant amount of matter in the visible universe exists in states of pressure and temperature inaccessible to many laboratory experiments - e.g. the interior of the large planets in our own solar system, and those that have been discovered orbiting other stars (exoplanets). Recreating and understanding the conditions existing in these regions is of fundamental interest. From the applied point of view it is as yet unknown what materials, once produced at high temperature and pressure, may be metastable (and of commercial use) at ambient conditions. For example, it is well known that diamond is metastable (it is not the state with the lowest free energy), however, the enthalpy barrier between it and the graphite phase is huge, and it thus exists under ambient conditions, and has considerable industrial impact. Thus the overall field of explore matter under novel conditions, learning its structure, and then bringing it back (if possible) to ambient conditions could be of considerable impact.

The aims of the project are to better understand, at the lattice level, the physics of high-strain rate plasticity and polymorphic phase transformations. This will be undertaken both by theoretical investigation (molecular dynamics simulations) and via experiments employing femtosecond x-ray diffraction of materials shock or ramp compressed by high power laser ablation. The aim is, for several classes of material, to better understand how, under the uniaxial strain conditions applied by laser ablation, the material relieves the shear stresses to flow towards the hydrostatic. We know that, depending upon the material, this can happen via dislocation generation and flow, via twinning, and via polymorphic phase transitions. However, what actually happens at the lattice level, and at the mesoscale of individual grains, has yet to be explored in any detail. In this project the student will study both simple metals and more complex targets to start to unravel this complicated set of phenomena.

This project is directly related to the laser-plasmas and fusion EPSRC theme. For example the physics is encompassed under EPSRC grant EP/J017256/1 The Creation and Diagnosis of Solid-State Matter at Multi-TeraPascal Pressures. This is 50% funded by AWE, and we will also collaborate closely with LLNL in the USA.

Publications

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Studentship Projects

Project Reference Relationship Related To Start End Student Name
EP/N509711/1 01/10/2016 30/09/2021
1963728 Studentship EP/N509711/1 01/10/2016 30/09/2019 Patrick George Heighway
 
Description With the aid of large-scale molecular dynamics simulations, we have investigated the role of interactions between neighbouring grains in polycrystals under the conditions of shock compression. Grain interactions have been afforded much consideration under quasistatic (extremely slow) loading conditions, but to date almost no attention has been paid to such interactions at high strain rate and high pressure. We have found that grain interactions can radically alter the manner in which each grain deforms in the wake of the shock. Adjacent grains are able to deform cooperatively via the huge stresses they exert on on another, and in so doing are able to reach a state of lower energy. We have also found that the stress changes precipitated by grain interactions can activate or deactivate certain deformation mechanisms in the polycrystal, depending on the shock pressure. In brief, our study has demonstrated the pervasive influence of grain interactions under extreme strain and pressure conditions.

We have further investigated by use of both molecular dynamics simulations and ultrafast x-ray diffraction techniques heating suffered by a metal as it releases from the shock state. Release is the fundamental process that takes place when a material at high pressure undergoes rapid decompression. It is commonly accepted that expansion of this sort takes place isentropically, meaning the material is expected to cool hugely as it releases. We have performed molecular dynamics simulations of micron-scale metal targets that exhibit post-release temperatures far exceeding what one would expect from an isentropic release. This observation has been corroborated by experiments in which the excessive release temperature of laser-shocked foils are deduced from their thermal expansion. The heating is found to result from the effects of material strength, which are completely neglected in the textbook picture of shock release. These results will thus challenge the conventional treatment of this fundamental thermodynamic process.
Exploitation Route It is our hope that our work will prompt further computational investigations of the dramatic grain-scale dynamics that take place under shock conditions, including such studies as are necessary to understand how best to go about detecting such dynamics experimentally using femtosecond x-ray diffraction techniques. We believe that the effects of grain interactions are sufficiently far-reaching that they are worthy of inclusion in any predictive model that seeks to recreate material behaviour to better than first order, and they certainly warrant further study in their own right.

We further anticipate that our experimental study of shock release will change the way such experiments are carried out and interpreted, and challenge the thermodynamic treatment of the process. The study should highlight how informative shock release (which has received far less attention than shock compression) can be, as we have shown that it essentially provides a proxy measurement of material strength during deformation at extreme pressure and strain rate.
Sectors Aerospace, Defence and Marine,Energy,Manufacturing, including Industrial Biotechology,Security and Diplomacy

URL https://journals.aps.org/prmaterials/abstract/10.1103/PhysRevMaterials.3.083602